1. Field of the Invention
The present invention relates to a heat treatment apparatus and a heat treatment method for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with flashes of light.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might present a problem in good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with flashes of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with flashes of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that the irradiation of a semiconductor wafer with flashes of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
U.S. Patent Application Publication No. US-2005-0063448-A1 discloses a technique in which a light measuring part in a flash lamp annealer includes a calorimeter disposed outside a chamber body, a light guide structure for guiding light emitted inside the chamber body to the calorimeter, and a calculating part for performing computations on the basis of an output from the calorimeter, the light measuring part being adapted to measure the energy of light emitted from flash lamps inside the chamber body by means of the calorimeter. U.S. Patent Application Publication No. US-2005-0063448-A1 also discloses a technique in which the surface temperature of a substrate is obtained by computations, based on the energy of flashes of light measured by means of the calorimeter.
The technique disclosed in U.S. Patent Application Publication No. US-2005-0063448-A1 presents the process of measuring the total energy (the total amount of heat) of a single flash of light to thereby determine the maximum temperature achieved at the surface of the substrate from the total energy ex post facto (i.e., after the irradiation of the substrate with flashes of light). In other words, the technique disclosed in U.S. Patent Application Publication No. US-2005-0063448-A1 is not capable of making real-time measurements of the surface temperature of the substrate which increases rapidly during the irradiation with flashes of light. Also, the technique disclosed in U.S. Patent Application Publication No. US-2005-0063448-A1, in which the temperature of the substrate surface is determined ex post facto after the irradiation with flashes of light, is not capable of controlling the irradiation of the substrate with flashes of light, based on the result of the measurements.
For high-precision flash lamp annealing, there is a need to make real-time measurements of the temperature of a substrate which increases during the irradiation with flashes of light. It is also desired that the irradiation of a substrate with flashes of light is controlled, based on the result of measurements of the temperature of the substrate.